This invention relates to a process for modifying the surface of fluoropolymers used in printed circuit boards and more particularly to a process utilizing a plasma and ammonia for modification of the polymer surface to allow one to plate the surface more efficiently by the use of electroless copper plating techniques.
As is ascertain, printed circuits are widely employed in the electronics industry. Essentially, a printed circuit is a conductive circuit pattern applied to one or both sides of an insulating substrate. The conductive pattern can be formed by any of several techniques after which component lead holes are drilled or punched out in the substrate and components are then installed and soldered in place. Printed circuit construction is ideal for assembly of circuits which employ miniature solid state components.
Its advantages over conventional chassis and point-to-point wiring are many and hence printed circuits are widely employed. In any event, many types of materials are utilized for printed circuit substrate materials and are available in thicknesses varying from 1/64 to 1/2 inch. Many printed circuit boards for particular application are fabricated with fluoropolymers.
Examples of such fluoropolymers are Teflon which is used in a polytetrafluoroethylene laminate type board (PTFE). Another type of board uses Kel-F which is a polymonochlorotrifluoroethylene compund. Other types of printed circuit boards, as one can ascertain, include Teflon film and so on. In any event, such fluoropolymers are characterized in having a very low surface energy and as such are not readily wetted by aqueous solutions or common inks, adhesives and coatings.
While the above-noted printed circuit boards are indicative of some of the fluoropolymers which are employed favorably with this process, it is indicated that the process to be described may be used with fluoropolymers such as Teflon, as indicated above, fluorinated ethylene-propylene (FEP), fluorinated alkoxyl (TFA), ethylene-tetrafluoroethylene copolymers such as Tefzel (ETFE) and other polymers and copolymers including fluorine including the following brands, Tedlar (Dupont, Kel-F (3 M), Halar (Allied) and Kynar (Pennwalt).
The above-noted materials are sold under trademarks, and essentially the company that manufactures or has the registration has been indicated in parenthesis. In any event, as will be described, the process to be described may be used whenever it is desired to wet such fluoropolymers or improve the adhesion of such materials as plating, inks, adhesives, glues, paints or other coatings. Essentially, the process may be used whenever it is desired to stick something to such materials as fluoropolymers.
In any event, the prior art was cognizant of a need to modify the surface of fluoropolymers such as Teflon and derived many techniques for doing so. One technique involves the use of elemental sodium dissolved or dispersed in a liquid such as a liquified ammonia or proprietary mixtures of solvents. These mixtures work by removing fluorine from the polymer and substitute oxygen and hydrogen when the surface is subsequently exposed to water.
Modification of the polymer surface in this manner allows materials such as plating solutions, adhesives and other coatings to wet the surface. This promotes void free plating and greater adhesion compared to the untreated surface. Certain companies such as the Rogers Corporation of Chandler, Ariz., have utilized what is referred to as a plasma dissmearing operation. In this operation boards are placed in a vacuum chamber. A mixture of fluorocarbon gas and oxygen or pure oxygen is introduced at low pressure. Pure oxygen plasma is used for providing wettability of PTFE. High electrical potential across the electrodes in the chamber ionizes the gas mixture to form a chemically active plasma that removes material from exposed polymeric surfaces.
Under proper conditions this exposure to the plasma will render PTFE surfaces water wettable which is an important characteristic for a successful hole in edge plating. See for an example an article entitled "Through Hole and Edge Plating of RT/duroid.RTM. Microwave Laminate", published by the above-noted corporation under document number 7397-1084-5K. In an article entitled "Keene's Low Loss Printed Circuit Board Laminates for the Microwave Industry" by John S. Taylor, 1984, published by the Keene Corporation, Laminates Division, 1100 Governor Lea Road, Bear, Del. 19701, there is described the treating of holes before plating through with electroless copper.
In the article it is indicated that since Teflon has a low surface energy, the resins do not wet out readily and the use of sodium etching is recommended. In the article it is described that both the sodium naphthelene in organic ethers and the sodium liquid ammonia solution are used for this purpose.
The former type is more convenient for the average user. The reagent may be produced by the user but is probably more convenient to purchase commercially prepared solution such as TETRAETCH from W. L. Gore or FLUOROETCH from Acton Associates. Sodium etching removes fluoride atoms from the surface of the polymer so that the chemically changed and somewhat carbonacious surface form becomes receptive to wetting by certain materials such as tin and paladium containing sensitizers or catalyst solutions. Roughening of holes in printed circuit boards by plasma etching has also been used instead of sodium etching before plating. Vapor honing before sodium etching has been reported to improve plating adhesion.
Hence as one can ascertain from the above-described prior art techniques there are many techniques utilized in the prior art to improve the wettability of fluoropolymers.
In any event, existing methods of modifying the surface of polymers involved the use of sodium as indicated above or employ liquified ammonia or other proprietary mixtures or solvents. The above processes have several short comings including cost, work place hazards and environmental hazards. Preparations of sodium are expensive and have limited shelf life. They give off noxious fumes that workers must be protected from. They are highly reactive and some proprietary mixtures are extremely flammable.
The used mixtures as well as the byproducts must be extensively treated before disposal or handled as a hazardous waste. Hence such techniques including the use of plasma techniques with the above-noted materials have created extremely dangerous byproducts and are expensive and time consuming to utilize.
It is therefore an object of the present invention to provide a process which utilizes a plasma and is capable of modifying the surface of fluoropolymers which operates at reduced pressures and uses gases as reactants. The gases used are oxygen and ammonia. These gases as will be explained are inexpensive and readily available.
It is a further object of this invention to provide a plasma process for modifying the surface of fluoropolymers which process is relatively inexpensive in operation and which process produces byproducts that are easily treated by scrubbing and ph neutralization.
It is a further object of the present invention to provide a process for modifying the surface of fluoropolymers which process is carried out in a vacuum chamber and hence is naturally isolated from the workplace. In this manner the preparation and storage of hazardous mixtures is avoided.